I. Field of the Invention
This invention relates, in general, to brazing filler metals and, more specifically, to filler metal compositions used to fill holes and repair damage in turbine engine high temperature components. In particular, the invention relates to novel filler metals which provide excellent high temperature corrosion and abrasion resistance. The invention further relates to a vacuum brazing process utilizing said filler metals.
II. Description of the Prior Art
Brazing consists of joining base metal surfaces by fusing a filler metal, having a lower melting point than the subject base metal, without appreciable fusion of the base metal surfaces themselves. For brazing, a flux may be applied to the subject base metal surfaces either prior to or simultaneously with the filler metal.
A satisfactory brazing flux flows at a temperature somewhat below the melting point of the filler metal; adheres to or wets the base metal surfaces; facilitates the flow and wetting of the filler metal over the subject base metal surfaces generally by reducing the surface tension of the molten filler metal; removes any oxide coating or other adherent foreign matter present on the subject base metal surfaces without appreciably attacking the base metal surfaces; inhibits re-oxidation of the subject base metal surfaces; and is capable of ready displacement by liquid filler metal either leaving no residue or leaving a readily removable, relatively inert residue after completion of the brazing.
Furnace brazing in a vacuum with the use of no flux offers several advantages. For example, the possibility of flux inclusions are eliminated and, accordingly, blind cavities, tortuous paths, and small passageways can be designed into the assembly without regard to flux removal or entrapment after brazing. In addition, fluxless vacuum brazing eliminates the cost of flux and its application, the need for cleaning the assembly after brazing, and potential corrosion of equipment and pollution of air and water by flux residues or flux reaction products.
Nickel-base, copper-base, gold-base, palladium-base, and a few silver-base filler metals are commonly used in vacuum furnace brazing. Apart from compatability with the base metal, filler metals are invariably selected for corrosion resistance in specific media and suitability for service at known operating temperatures.
Known brazing filler metal compositions, however, do not have the desired properties necessary for use in filling small holes and other defects in high temperature superalloys such as those used in turbine engine high temperature components. As a result, engines with small holes therein lose efficiency and parts must be scrapped. Moreover, known filler metals do not simultaneously give good wetting, but very limited flow and the ability to bridge defects, at a brazing temperature of about 1,950.degree. F., so that defects are sealed without filler material flowing into internal passages in the components. In addition, known filler metals do not have the proper wetting and flow characteristics at a brazing temperature of 1,950.degree. F. while also possessing the ability to fill and bridge holes as well as to provide excellent high temperature and corrosion resistance and, when properly coated, survive in the harsh environment of a turbine engine.
It is, therefore, an object of this invention to provide a brazing filler metal composition which is devoid of the above-noted disadvantages.
It is another object of this invention to provide a brazing filler metal composition which has desired properties for use in filling small holes and other defects in high temperature superalloys, such as those used in turbine engine high temperature components.
It is still another object of this invention to provide brazing filler metals which wet well and yet have very limited flow at a brazing temperature of about 1,950.degree. F. while possessing the ability to fill and bridge holes.
It is yet another object of this invention to provide brazing filler metals which provide very good high temperature corrosion and abrasion resistance.
It is still another further object of this invention to provide brazing filler metal compositions wherein the brazing process may be accomplished in a single pass.
It is yet another object of this invention to provide a brazing filler metal which may be overcoated with coating schemes used for high temperature nickel superalloys.
It is yet another further object of this invention to provide a vacuum brazing process utilizing novel nickel-base filler metal compositions.